[Potential bug found] cannot borrow `xxx` as mutable more than once at a time when using mpd's Query::and

[CoorFun]

Hello,

Please refer to my stackover flow question. The first answer found a potential bug.

https://stackoverflow.com/questions/58878192/cannot-borrow-xxx-as-mutable-more-than-once-at-a-time-when-using-mpds-query/58879004#58879004

[SimonTeixidor]

I created a PR to fix this: #47.